1. Field of the Invention
The present invention relates to improvements in .lambda. sensors for detecting the air/fuel ratio or equivalent ratio .lambda. of exhaust gases, and more particularly to a .lambda. sensor utilizing variations in the resistance value of a metallic oxide semiconductor. The .lambda. sensor of the present invention is suitable for controlling internal combustion engines such as motor vehicle engines and for controlling combustion devices such as air heaters, boilers and the like.
2. Prior Art
The present inventor has discovered that compounds such as BaSnO.sub.3-.delta., RaSnO.sub.3-.delta., CaSnO.sub.3-.delta. and SrSnO.sub.3-.delta. exhibit outstanding characteristics as .lambda. sensors (Japanese Patent Applications SHO No. 59-63900 and SHO No. 59-63901, corresponding U.S. patent application Ser. No. 711,154 and corresponding European Patent Application No. 85103,512). These compounds have the crystalline structure of the perovskite type and are novel .lambda. sensor materials. They have the features of being highly durable against high-temperature reducing atmospheres and highly sensitive to variations in oxygen partial pressure. They have another feature in that the sensitivity to combustible gases and the sensitivity to oxygen are relatively in good balance.
To mention related prior-art references, Examined Japanese Patent Publication SHO No. 50-23317 (corresponding to U.S. Pat. No. 3,644,795 and West German Patent No. 2,062,574) discloses that the addition of silica binder to SnO.sub.2 affords a gas sensor of improved mechanical strength without impairing air permeability.
U.S. Pat. No. 4,225,559 further discloses that the addition of Pt-Rh catalyst to TiO.sub.2 provides a sensor of improved responsiveness to changes of atmosphere.